1. Field of the Invention
This invention relates to semiconductor processing and, more particularly, to furnaces used for semiconductor processing.
2. Description of the Related Art
During semiconductor manufacturing, vertical furnaces are commonly used to subject semiconductor substrates, such as wafers, to a number of processing steps, including oxidation and diffusion steps, at elevated temperatures. A heat resistant material in the shape of a cylindrical tube can be used to form the interior walls of the furnaces. The lower end of a vertical furnace reaction tube is typically open to allow loading and unloading of a substrate holder, e.g., a wafer boat, which typically holds a plurality of substrates. During processing, the lower end of the reaction tube can be sealed by a movable closure.
An example of a vertical furnace 103 is shown in FIGS. 1 and 2, wherein identical parts are indicated by identical reference numbers. With reference to FIG. 1, a process tube 100 is supported at its lower end on flanges 130. The open bottom end of the process tube 100 can be closed by positioning a door plate 140 against the process tube 100. A preferred material for the process tube 100 and the door plate 140 is quartz, because of its purity and high temperature resistance. When closed, the process tube 100 and the door plate 140 delimit a reaction, or process, chamber 104. It will be appreciated that process gas flows into the reaction chamber 104 through process a gas inlet 102 and then flows downward and exits the chamber 104 out of a gas exhaust 136, which is a gas passage in the process tube 100 near the open end of the tube 100.
The quartz door plate 140 rests on three or more feet 142, which in turn rest on a metal door plate 150. In the illustrated example, the metal door plate 150 is provided with a rotation bearing 153, which allows a wafer boat 112 to rotate during processing. Part 152 is the stationary part and part 154 is the rotating part of the rotation bearing 153. A rotating support plate 144 rests on the rotation bearing 153 and supports a pedestal 110, which in turn supports the wafer boat 112.
With reference to FIG. 2, closing the process tube 100 involves forming a quartz-to-quartz seal between the process tube 100 and the door plate 140. The door plate 140 comprises an upper sealing surface 141 that abuts a lower sealing surface 101 of the process tube 100 and, thus, forms a quartz-to-quartz seal. However, quartz contacting quartz does not make for a completely airtight seal. Aggressive process chemicals, such as HCl, can diffuse through the quartz-to-quartz seal and undesirably contact and corrode the metal door plate 150 and/or leak out of the furnace 103.
To minimize this corrosion and gas leakage, a lower sealing chamber 106 can be formed at the lower end of the process chamber 104, immediately below the quartz door plate 140. The lower wall of the sealing chamber 106 is formed by the metal door plate 150 and the sealing chamber 106 is sealed from the outside world by resilient sealing members such as O-rings or lip-seals 156.
Inert purge gas is flowed into the sealing chamber 106 through an inlet 134 to expel process gases that diffuse through the quartz-to-quartz seal and to prevent process gases, such as HCl, from reaching the metal door plate 150 and corroding the metal material of the plate 150. The flow of a purge gas, e.g., N2, is shown by arrows, starting from the inlet 134. The purge gas flows through the sealing chamber 106 in a generally radially inward direction until, in a center region of the quartz door plate 140, the purge gas leaves the sealing chamber 106 and enters the process chamber 104 by flowing upward and radially outward past a flow restriction 146, which restricts the flow of gas by decreasing the cross-sectional area of the passage through which the gas flows. Then, the purge gas flows in the direction of an exhaust 136.
While effective at minimizing door plate corrosion and process gas leakage, flowing inert gas through the sealing chamber 106 has been observed to detrimentally affect process results on substrates processed in the reaction chamber 104.
Accordingly, there is a need for a furnace closure and exhaust construction that allows purging of a sealing chamber, while also minimizing the effects of the purge on process results.